There was studied the composition of periphyton and its quantitative development during different stages of wastewater treatment in the flow type purification plant with the biological part working as a modified bardenpho system (Hajdow, Lublin). The periphyton samples were taken from the walls of the object at all main stages of water purification. In the composition of periphyton there were identified the following groups of organisms: algae, fungi, flagellates, testate amoeba, ciliates, rotifers and nematodes. Towards the end of purification process in periphyton of successive chambers, the part of metazoa and protozoa has increased while the abundance of flagellates has decreased. In all of the studied sampling points, the basis of periphyton community was formed by protozoa and metazoa: their part ranged from 75 to 95 %. The periphyton structure regardless of forming conditions shows similar tendencies, determined by the general conditions of a system, in which from initial stages of the purification till its end the amount of organic matters in environment decreases.